Epoxies serve massive global markets in adhesives and coatings, and are also one of the industry standard thermosetting plastic matrices used for construction of fiber reinforced plastics (FRP). FRPs are composite materials consisting of a polymer matrix and a fiber such as carbon fiber, fiberglass, aramid fiber, natural fiber, or other fiber. The fiber serves to enhance the properties of the plastic in areas such as strength and elasticity. FRPs are also commonly referred to as “plastic composites” or, for simplicity, just “composites.” The term “plastic composites” can also embody plastic materials that have non-fibrous entities incorporated in them such as metals or nanomaterials. Plastic composites provide lightweight alternatives to other structural materials (e.g., steel or aluminum) and are widely used in the automotive, aerospace, nautical craft, wind energy, and sporting goods sectors. The incorporation of lightweight composites can offer substantial environmental benefits by way of leading to increased energy efficiency; yet, the positive impact of thermosetting plastic composites is offset by their lack of recyclability and persistence in the environment. The predicted waste accumulation in the growing wind power industry is an illustrious example. The current output of wind energy is approximately 10 times that of the production in 1980, and windmill blade propellers can reach over 60 meters in length. The material wastage from wind motor blade is estimated to reach 225,000 tons per year by 2034. The weight percentage of epoxy in fiber reinforced epoxies typically is in the range of 25-40%. The raw materials (i.e. the plastic and fiber) that go into composite construction can be expensive, and are usually of petrochemical origins. Thus, there are both economic and environmental drivers for the development of new recyclable fiber reinforced epoxy plastics.
The most common epoxy formulations consist of a diepoxide (“resin”) and a polyamine (“hardener”) to form a cross-linked polymeric network of essentially infinite molecular weight (the combination of “resin+hardener” is sometimes referred to as “cured epoxy” or “cured resin” or simply “resin” or epoxy). The widespread utility of such epoxy formulations for composite manufacturing is due to excellent processablity prior to curing and their excellent post-cure adhesion, mechanical strength, thermal profile, electronic properties, chemical resistance, etc. Further, the high-density, three-dimensional network of epoxies make them extremely robust materials, tolerant of a wide range of environmental conditions. At the same time, the cross-linked network makes their removal, recycling and reworkability notoriously difficult. The cross-linking reactions that occur with conventionally used polyamine epoxies formulation are essentially irreversible; therefore, the material cannot be re-melted and re-shaped without decomposition; the material cannot be readily dissolved either. As a result, finber reinforced epoxies or epoxy-based composite materials are not amenable to standard recycling practices because the epoxy matrix and fibers cannot be readily separated, and recovered.
Current disposal methods of composites typically involve land filling, grinding and burning. Burning provides a mechanism to recover some of the energy input, but the incineration process requires large amounts of energy and remains questionable from an environmental standpoint. An emerging technology for recycling of carbon fiber composites involves special incinerators that are capable of burning away the plastic matrix of the composite and leaving behind the carbon fiber, which then can be reclaimed. While this approach is a step forward with regard to sustainability, it does not represent a more fully recyclable approach as the plastic matrix is not recovered in a repurposable form as it is destroyed in the process.
There are no known examples in the prior art of use of composite materials constructed from reworkable epoxy compositions. The use of reworkable and/or degradable epoxy resin composition for the fabrication of composite materials is unknown in the art. Further, the recycling of degradable epoxy composites, whereby the constitution of the epoxy and the reinforcement material are recovered with high efficiency.